Research on the role of educational systems in educational outcomes has received increasing attention in recent decades. Focusing on the influences of educational systems on Inequalities in Educational Opportunities, this contribution asks how different levels of stratification, selectivity and standardization contribute to social and ethnic inequalities in educational attainment. More precisely, educational attainment is understood as the consequence of learning or achievement acquisition on the one hand, and educational decision-making on the other. The partly contradictory results of educational institutions on these two outcomes, which are presented in the empirical part of the contribution, underline the need for greater conceptual and theoretical rigour when investigating the role of educational institutions on educational inequalities. Instead of focusing more generally on inequalities, it should be made clearer which kind of inequality may be affected by educational institutions, that is, learning or educational decision-making. Furthermore, the need for more complex theoretical considerations and the necessity to focus on the underlying mechanisms of how institutional settings lead to educational inequalities are further underlined by the partly contradictory results being found for social as well as ethnic inequalities on both dimensions.